1. Field of the Invention
The present invention relates to the field of synchronous radio telecommunications. In particular it concerns a method and a corresponding device which is able to transmit/convert synchronous signals with a bit rate significantly below that of an STM-1 signal.
2. Description of the Prior Art
As it is known from ITU-T Recommendation G.707, a synchronous transport module (STM) is the information structure used to support connections at section layer in the SDH (Synchronous Digital Hierarchy) transmissions. Each STM comprises Payload and Section Overhead (SOH) information fields organized in a block frame structure which repeats every 125 μs. The information is suitably conditioned for serial transmission on the selected media at a rate which is synchronized to the network. A basic STM structure is defined at 155,520 Kbit/s and is termed STM-1. Each STM-1 comprises a single AUG (Administrative Unit group) together with the Section Overhead.
In various network practical applications, it is not necessary to transport a complete STM-1 and often the bandwidth of the radio channels is too small to support full STM-1 transmission. Therefore, for applications based on such limited-band radio channels, a synchronous signal format with a bit rate significantly below an STM-1 signal is useful.
Within the activity carried out by the standardization entities (ITU-R and ETSI) operating in the field of radio communication, such a signal format has been developed and the approval of ITU-T has been achieved. The signal in question, termed “sub-STM-1 signal”, provides one third of the capacity of an STM-1 signal and maintains most of the benefits of the synchronous transmission. In essence, an interface called “Sub-STM-1” is provided which allows the transport of a VC-3 (Virtual Container-3) in an AUG based upon an AU-3 (Administrative Unit-3).
This new sub-STM-1 interface is not a Network Node Interface (NNI) in ETSI market and thus it is necessary for the radio network element (NE) to have an STM-1 interface and the capability to perform an STM-1 to sub-STM-1 signal conversion to transmit over radio channel. Naturally, the STM-1 signals in these instances are considered as only partially filled i.e. they carry only an “equipped” VC-3.
It is therefore apparent that with this known solution a radio NE must carry out a series of operations among which: a demultiplexing operation with an ADM or a DEMUX; a pointer processing operation through an appropriate algorithm; a parity calculation operation; and, lastly, a new multiplexing operation to sub-STM-1. Clearly, all these operations cannot be carried out in a “pure regenerator” NE and an ADM or MUX/DEMUX apparatus should be added.
In other words, the existing solution for interconnecting an STM-1 with a sub-STM-1 utilizes the interconnection rules between AUGs based upon a different type of Administrative Unit AU-4 and AU-3. This method requires that the AU-4 is demultiplexed to the Virtual Container VC-3 or TUG-2 level according to the type of the payload, and remultiplexed within an AUG via VC-3/AU-3 path. The known method is illustrated in FIGS. 1a (interconnection of VC-3 with C3 payload) and 1b (interconnection of TUG-2), as in the ITU-T Recommendation G.707.